1. Field of the Invention
The present invention is generally related to the field of communication devices. More particularly, the present invention is related to echo cancellation in communication devices.
2. Related Art
The increasing demand for high-speed Internet connectivity has in turn increased the demand for various communication devices such as modems, and in particular for modems utilizing digital subscriber line (“DSL”) technology which provides high-speed transmission of voice, video, and data information over existing copper telephone lines. Although the DSL modem is used merely as an example in the present application to illustrate shortcomings of the existing art, such shortcomings and the solutions set forth by the present invention, are also applicable to other communication devices.
DSL technology utilizes a DSL modem to achieve data speed from 128 Kbps to 8 Mbps and higher over an ordinary telephone line. During operation, the DSL modem utilizes the same line for both transmitting and receiving information. As a result, the signal transmitted by a line driver in the DSL modem can encounter discontinuities in the line, which can cause part of the transmitted signal to reflect or echo back and interfere with a received signal. In an effort to eliminate interference to the received signal that is caused by the echo of the signal transmitted by the line driver, DSL modem manufacturers employ echo cancellation circuitry.
To achieve echo cancellation, DSL modem manufacturers typically employ a hybrid circuit comprising analog components. By utilizing a hybrid circuit, an echo reduction or attenuation of approximately 15.0 to 20.0 dB can be achieved. However, proper reception of the received signal requires that the signal strength of the echo be attenuated below the signal strength of the received signal. Thus, since the signal strength of the received signal can be approximately 80.0 dB below the signal strength of the transmitted signal, an echo reduction of 15.0 to 20.0 dB does not provide sufficient echo cancellation for proper reception of the received signal. Greater echo reduction can be achieved in an external hybrid circuit by utilizing very accurate components and very accurately matching the impedance of the line. However, very accurate components are costly and matching the impedance of the line to a required degree of accuracy is difficult to achieve.
To achieve further echo cancellation, DSL modem manufacturers utilize a digital echo canceller (“DEC”) in the DSL modem. By utilizing a DEC, a desirable echo attenuation or reduction of approximately 80.0 dB can be achieved. However, the DEC is essentially a linear echo canceller and, as such, works very well as long as the line driver that transmits the output signal of the DSL modem is linear.
During actual operation, the signal transmitted by the line driver includes non-linear signal components, since linearity is difficult to achieve in a high power and high current device such as a line driver. As a result, the echo will include both linear and non-linear signal components. Thus, since the DEC is essentially a linear echo canceller, the DEC will only cancel the linear component of the echo. Since the hybrid circuit can reduce both the linear and non-linear components of the echo, the hybrid circuit can be utilized to attenuate the non-linear component of the echo. However, the hybrid circuit typically attenuates the non-linear component of the echo by only approximately 15.0 to 20.0 dB. Thus, the unattenuated portion of the non-linear component of the echo will be passed through to the decoder portion of the DSL modem as noise, which can undesirably interfere with reception of low level signals.
Although the DEC could be designed to cancel the non-linear portion of the echo, the complexity of the DEC would be greatly increased, since an exponentially increasing amount of digital circuitry would be required for each harmonic of the non-linear portion of the echo. As a result, the cost of the DEC would be undesirably increased.
Thus, there is a need in the art for a cost-effective system for cancellation of linear and non-linear echo components created by a line driver in a communication device, such as a DSL modem.